The present invention relates to a method of manufacturing a cathode-ray tube (CRT).
In cathode-ray tubes, in each grid, an electron gun in which each grid is supported by a pair of glass beads is sealed into a neck portion.
A cathode-ray tube is treated by a process for preventing a potential within a CRT-assembly from being fluctuated due to stray charges charged on the inner wall of the neck portion opposing the CRT-assembly and the surface of glass bead on application of high voltage.
In this treatment, a metal ribbon serving as a metal strap, e.g., thin stainless steel material having a width of 0.1 mm and a thickness of 0.1 mm is wound around a part of a pair of glass bead, the thin stainless steel material is heated from the outer periphery of the neck portion by using a high-frequency (or radio-frequency) induction heating means and evaporated, and a metal deposited film is deposited on the inner wall surface of the neck portion of the corresponding portion and the surface of the glass bead.
The same assignee of this application has previously proposed a color cathode-ray tube shown in FIGS. 1 through 3.
As shown in FIG. 1 in an enlarged-scale, an electron gun 2 of a cathode-ray tube 1 comprises three cathodes K.sub.R, K.sub.G and K.sub.B corresponding to red (R), green (G) and blue (B) arranged in line, a first grid G.sub.1, a second grid G.sub.2, a third grid G.sub.3, a fourth grid G.sub.4, a fifth grid G.sub.5, a sixth grid G.sub.6 and a seventh grid G.sub.7 common to the three cathodes K.sub.R, K.sub.G, K.sub.B sequentially arranged and three beam apertures 3.sub.R, 3.sub.G, 3.sub.B for passing electron beams emitted from the three cathodes K.sub.R, K.sub.G and K.sub.B defined in the first through seventh grids G.sub.1 to G.sub.7.
The first grid G.sub.1 is applied with a voltage of 0 V, the second grid G.sub.2 and the fourth grid G.sub.4 are connected commonly and applied with a voltage of 700 V, the third grid G.sub.3 and the fifth grid G.sub.5 are connected commonly and applied with a voltage of 6 kV, the sixth grid G.sub.6 is applied with a voltage ranging from 6 kV to 6.5 kV and the seventh grid G.sub.7 is applied with a voltage of 25 kV which is an anode voltage, thereby resulting in the electron gun 2 being arranged as a bi-potential type electron gun.
Electron beams emitted from the cathodes K.sub.R, K.sub.G and K.sub.B are converged on a fluorescent screen (not shown) through the beam apertures 3.sub.R, 3.sub.G, 3.sub.B of the grids G.sub.1 through G.sub.7.
As shown in FIG. 2, the grids G.sub.1 through G.sub.7 are integrally supported by a pair of glass beads 4 and 5 and this electron gun 2 is sealed into a neck portion IN of the cathode-ray tube 1.
When a high-voltage supplying contact member 6 integrally elongated from the seventh grid G.sub.7 is brought in contact with an inner carbon film 7 connected to an anode button (not shown), the seventh grid G.sub.7 is applied with an anode voltage of 25 kV, for example.
On the other hand, the voltage of 6 kV is applied to the third grid G.sub.3 and the fifth grid G.sub.5 through a voltage-dividing resistor 9. As shown in FIGS. 1 and 3, this voltage-dividing resistor 9 is formed such that an internal resistor 11 is formed on a ceramic base 10, electrode terminals t.sub.1, t.sub.2 and t.sub.3 are formed on respective ends and an intermediate portion, the internal resistor 11 is coated with an insulating glass layer 12 except the terminals t.sub.1, t.sub.2 and t.sub.3 and that the rear surface of the ceramic base 10 also is coated with the thin glass layer 12.
The voltage-dividing resistor 9 is disposed on one glass bead 4, the first electrode terminal t.sub.1 thereof is connected to the seventh grid G.sub.7, the second electrode terminal t.sub.2 thereof is connected to the earth terminal, and the intermediate third terminal t.sub.3 is connected through a common connection member 13 to the third grid G.sub.3 and the fifth grid G.sub.5.
In the cathode-ray tube 1 in which the above-mentioned electron gun 2 is sealed, metal straps 15 and 16 are wrapped around the electron gun 2 at its glass beads 4 and 5 on the portion corresponding to the fifth grid G.sub.5, for example. One metal strap 15 is wound around the glass bead 4, including the voltage-dividing resistor 9, and the other metal strap 16 is wound around only the glass bead 5.
As shown in FIGS. 5 and 6, a radio-frequency induction heating means, i.e., radio-frequency heating coil 18 is disposed around the neck tube IN at its outer periphery corresponding to the metal straps 15 and 16. When this radio-frequency heating coil 18 is energized by a radio-frequency induction current 19, the radio-frequency heating coil 18 generates a uniform magnetic flux 20 so that an induction current is flowed to the metal straps 15 and 16 to heat and evaporate the metal straps 15 and 16. As a consequence, as shown in FIG. 7, metal deposited films 21 and 22 are formed on the neck portion IN at its portions corresponding to the inner wall, the surfaces of the glass beads and the surface of the voltage-dividing resistor. In this case, the metal deposited films 21 and 22 should be deposited in such a manner that the metal straps 15 and 16 may not be blown out by evaporation.
Since one metal strap 15 is wound around the glass bead 4 and the voltage-dividing resistor 9 and the other metal strap 16 is wound around only the glass bead 5 due to the structure of the electron gun 2, the metal straps 15 and 16 are not symmetrical and the portions which are in contact with the metal straps 15 and 16 are different in thermal conductivity. In other words, the metal strap 15 contacts with the glass bead 4 and the ceramic base 10 and the metal strap 16 contacts with only the glass bead 5 so that the metal deposited films 21 and 22 are not deposited symmetrically and uniformly.
Specifically, the metal straps (15, 16) are brought in contact with the surfaces of the glass beads and the ceramic base whose thermal conductivities are changed with a rise of temperature. As a consequence, since heat releases of metal straps are different, the metal straps reach a deposition temperature with different times, i.e., the metal strap having only the glass bead reach the deposition temperature earlier than the other metal strap. Thus, the metal deposited films 21 and 22 are not deposited uniformly and symmetrically.
Therefore, a freedom is small from a condition standpoint, one of metal deposited films is not deposited or one metal strap is blown out and cut.